Flexographic printing is widely used in the production of newspapers and in the decorative printing of packaging media. Photosensitive printing plates and cylindrical printing sleeves have been developed to meet the demand for fast, inexpensive processing and long press runs.
Solid photocurable elements can exist as cylinders or rectilinear sheets. The photocurable element comprises a substrate, one or more photocurable layers, and often a protective cover sheet. The protective cover sheet is formed from plastic or any other removable material that can protect the plate or photocurable element from damage until it is ready for use. The photocurable element can optionally comprise a slip film disposed between the protective cover sheet and the photocurable layer(s) to protect the plate from contamination, increase ease of handling, and act as an ink-accepting layer.
The use of a photosensitive printing medium for the manufacture of flexographic printing elements, including plates and sleeves, is described in general terms as follows. The photosensitive resin printing material is deposited onto a substrate to form a printing element. The photosensitive resin may be deposited onto the substrate in a variety of ways, e.g., by extrusion, roll coating, heat processing, solvent casting, and the like. These techniques can be readily carried out by those skilled in the art.
The desired image is produced on the printing plate by exposing selected portions of the resin to actinic radiation. Selective exposure of the photosensitive resin can be achieved for example, by the use of an image-bearing transparency such as a negative film on the surface of the photosensitive layer, through the front side of the photosensitive resin. Areas of the transparency opaque to actinic radiation prevent the initiation of free-radical polymerization within the photosensitive layer directly beneath the transparency. Transparent areas of the image-bearing element will allow the penetration of actinic radiation into the photosensitive layer, initiating free-radical polymerization, rendering those areas insoluble in the processing solvent. Alternatively, exposure of selected portions of the photosensitive layer to laser radiation or other focused radiation beam may also initiate free-radical polymerization, rendering those areas insoluble in the processing solvent.
The unexposed and therefore unhardened portions of the resin are selectively removed by using a development process such as washing in a suitable solvent or thermal blotting. Washing may be accomplished by a variety of processes, including brushing, spraying, or immersion. The resulting surface has a relief pattern that reproduces the image to be printed. The printing element is mounted on a press and printing commences.
The support sheet, or backing layer, can be formed from a transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred substrate materials include steel, copper, or aluminum sheets, plates, or foils; paper; or films or sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefins, polyamides, and the like. The support sheet can optionally comprise an adhesive layer for more secure attachment to the photocurable layer(s).
The photosensitive layer(s) can include a variety of known materials, such as polymers, initiators, reactive diluents, fillers, and dyes. Preferred photosensitive compositions include an elastomer compound, an ethylenically unsaturated compound having at least one terminal ethylenic group, and a photoinitiator. Such materials are described in numerous patents and publications and are well known to those skilled in the art.
The photosensitive materials of the invention should cross-link (cure) and, thereby, harden in at least some actinic wavelength region. As used herein, actinic radiation is radiation capable of effecting a chemical change in an exposed moiety. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and violet wavelength regions. Preferred actinic wavelength regions are from about 320 nm to about 450 nm, more preferably from about 355 nm to about 415 nm.
As discussed above, photosensitive resin compositions generally cure through radical polymerization, upon exposure to light. The curing reaction is inhibited by oxygen, which is dissolved in the resin compositions, because oxygen functions as a radical scavenger. It is therefore highly preferred that the dissolved oxygen be removed from the photosensitive resin composition prior to exposure. Various techniques have been suggested for removing dissolved oxygen from the photosensitive resin composition. For example, the photosensitive resin composition may be placed in an atmosphere of inert gas (e.g. carbon dioxide or nitrogen) overnight before exposure in order to replace the dissolved oxygen with the inert gas by way of diffusion. The drawback to this method is that it can take a long time and requires a large space for the necessary machinery.
Alternatively, the photosensitive resin printing element may be given a weak blanket “pre-exposure” to consume the dissolved oxygen prior to subjecting the printing element to the main image-wise exposure. This pre-exposure step is often called a “bump” exposure. The bump exposure is applied to the entire plate area and is a short, low dose exposure of the plate that ostensibly reduces oxygen, which inhibits photopolymerization of the plate (or other printing element). Without this pre-sensitization step, fine features (i.e., highlight dots, fine lines, isolated dots, etc.) are not preserved on the finished plate. However, the pre-sensitization step tends to cause shadow tones to fill in, causing the printed gamut to be significantly reduced. This is exacerbated in plate formulations that have very high sensitivity and small exposure latitude. An additional drawback to this method is that the bump exposure requires specific conditions, including exposure time, irradiated light density, and the like, so that only the dissolved oxygen is quenched.
The pre-sensitization effect also wears off as the elapsed time between the bump exposure and main exposure increases. In conventional exposure of a printing plate, the elapsed time between the bump exposure and the main exposure is typically greater than about 20 seconds, allowing some oxygen to re-enter the plate prior to the main exposure. This delay causes the finished plate to have deep shadows. On the other hand, if the main exposure is applied very soon after the pre-sensitization step, as is envisioned in a computer-to-plate process, the tendency of shadow tones to fill is further worsened in comparison with conventional exposure techniques.
Other efforts to improve the relief image printing plate have involved special plate formulations alone or in combination with the bump exposure.
For example, U.S. Pat. No. 5,330,882 to Kawaguchi et al., incorporated herein by reference in its entirety, describes a photosensitive resin composition that comprises a polymer binder, a radically polymerizable monomer, a sensitizing dye, and a polymerization initiator wherein the preliminary (bump) exposure is conducted with a light that only excites the sensitizing dye and the main exposure is conducted with a light that excites the photopolymerization initiator. In this instance, the preliminary exposure is conducted with a light only exciting the sensitizing dye, and the main exposure is conducted with a light exciting the photopolymerization initiator.
U.S. Pat. No. 4,540,649 to Sakurai, incorporated herein by reference in its entirety, describes a photopolymerizable composition that contains at least one water soluble polymer, a photopolymerization initiator and a condensation reaction product of N-methylol acrylamide, N-methylol methacrylamide, N-alkyloxymethyl acrylamide or N-alkyloxymethyl methacrylamide and a melamine derivative. According to the inventors, the composition eliminates the need for pre-exposure conditioning and produces a chemically and thermally stable plate.
U.S. Pat. No. 5,645,974 to Ohta et al., incorporated herein by reference in its entirety, discloses a photocurable mixture that includes paraffin or a similar waxy substance to inhibit the effect of atmospheric oxygen. Due to its low solubility in the polymer, the paraffin floats at the beginning of the polymerization and forms a transparent surface layer that prevents the ingress of air.
Although various methods of inhibiting/removing dissolved oxygen in the photosensitive resin composition have been suggested, there remains a need in the art for an improved method of removing dissolved oxygen, especially in computer-to-plate (CTP) processes.